Effect of Reactant Injection Rate on Solidifying Aeolian Sand via Microbially Induced Calcite Precipitation

Aeolian sand is a type of special soil with a loose structure, fine and uniform particles, and poor self-stabilization ability. These characteristics easily lead to sand dune movement and wind erosion in dry desert conditions. Therefore, reinforcement technology for aeolian sand is an important research topic. Microbially induced calcite precipitation (MICP) is a novel microbial soil-strengthening technique considered in this study with a focus on the effect of the reactant injection rate on the microbial solidification of aeolian sand. The aeolian sand was solidified using MICP with different injection rates of the cementing solution. The physical and mechanical properties as well as the microstructure of the solidified aeolian sand samples were analyzed. The experimental results show that using Sporosarcina pasteurii and the cementing solution (a mixture of urea and calcium chloride) can effectively solidify aeolian sand using the MICP technology. The engineering performance of the sand was effectively improved with an unconfined compressive strength that approached 14.01 MPa. The permeability coefficient of the solidified aeolian sand was significantly reduced, and the injection rate of the cementing solution significantly affected its uniformity. When the injection rate of the cementing solution was too low, most of the calcium carbonate was generated and accumulated at the top of the sand sample. Therefore, the upper part of the sample was relatively dense and strong, while the lower part had the opposite properties. When the injection rate of the cementing solution was too high, the roles of the upper and lower parts of the sample were reversed. When the injection rate of the cementing solution was $0.278{\mathrm{mol}\mathrm{L}}^{-1}{\mathrm{h}}^{-1}$, the solidified aeolian sand sample was relatively uniform and had a moderate unconfined compressive strength of $4.58–5.03\mathrm{MPa}$. The uniform and high strength of the solidified aeolian sand was obtained by controlling the reactant injection rate. The optical microscope and SEM analyses indicated that the calcium carbonate crystals were rhombic hexahedral with sizes of approximately $5–10\mathrm{\mu m}$. The calcium carbonate crystals were generated from the MICP in the solidified aeolian sand, which cemented the sand particles together, filled the pores between particles, increased the density and strength of the sand, and reduced its permeability coefficient.